What are El Niño and La Niña phenomena and how do they change the weather?

What are El Niño and La Niña phenomena and how do they change the weather?

  • Written by Mark Pointing and Esme Stallard
  • BBC News Climate and Science

Image source, Getty Images

The Australian Bureau of Meteorology said the weather phase of El Niño, which began in the summer of 2023, has now ended.

This has helped push global temperatures to record levels in recent months, on top of long-term human-caused warming.

What is the El Niño phenomenon?

El Niño is part of a natural climate phenomenon called the El Niño Southern Oscillation (ENSO).

It has two opposing conditions: El Niño and La Niña, both of which dramatically change global weather.

El Niño can be identified by a number of different measurements, including:

  • Sea surface temperatures are warmer than normal in the eastern tropical Pacific
  • Above normal air pressure in Darwin, Australia (Western Pacific Ocean), and below normal air pressure in Tahiti, French Polynesia (Central Pacific Ocean)

Under “neutral” conditions, surface waters in the Pacific Ocean are cooler in the east and warmer in the west.

The “trade winds” tend to blow from east to west, and the sun's heat gradually warms the water as it moves in that direction.

During El Niño events, these winds weaken or reverse, sending warm surface water eastward instead.

During La Niña periods, normal east-to-west winds become stronger, pushing warm water westward.

This causes cold water to rise – or “rise” – from the deep ocean, meaning sea surface temperatures are cooler than normal in the eastern Pacific.

This phenomenon was first observed by Peruvian fishermen in the 17th century, who noticed that warm waters peaked near the Americas in December.

They called it “El Niño de Navidad” – the Christ Child in Spanish.

How do El Niño and La Niña change the weather?

Not all events are the same, and the consequences vary from one region to another.

However, scientists noticed some common effects:

Global temperatures typically rise during El Niño and fall during El Niño.

The El Niño phenomenon means that warm water circulates more and stays closer to the surface. This releases more heat into the atmosphere, creating more humid and warmer air.

But regional influences are complex, and some places may be warmer and colder than expected at different times of the year.

During El Niño events, warm waters push the strong air currents of the Pacific Jet Stream south and east.

This brings wet weather to the southern United States and the Gulf of Mexico.

Tropical regions such as Southeast Asia, Australia, and Central Africa typically experience drier conditions.

Under the La Niña phenomenon, the effect reverses.

El Niño also affects atmospheric circulation patterns, meaning there are generally more tropical storms in the tropical Pacific, but fewer in the tropical Atlantic, including the southern United States.

During La Niña, the opposite is usually true.

Carbon dioxide (CO2) levels.

If plants grow less quickly due to drought, they absorb less carbon dioxide, while more forest fires in places like South Asia mean more carbon dioxide is released.

Why are El Niño and La Niña climate patterns important?

Extreme weather events exacerbated by El Niño and La Niña impact infrastructure, food and energy systems around the world.

For example, when less cold water rises to the surface off the west coast of South America during El Niño events, fewer nutrients rise from the ocean floor.

This means that there is less food available for marine species such as squid and salmon, which in turn leads to reduced stocks for fishing communities in South America.

Image source, Getty Images

Comment on the photo, Fishing stocks can be affected by the effects of El Niño

How often do El Niño and La Niña occur?

El Niño and La Niña episodes typically occur every two to seven years, and usually last nine to 12 months.

They do not necessarily alternate: La Niña events are less common than El Niño episodes.

Does climate change affect El Niño/La Niña?

In 2021, UN climate scientists, IPCC, said that ENSO events that have occurred since 1950 are stronger than those observed between 1850 and 1950.

But she also said tree rings and other historical evidence show there have been variations in the frequency and strength of these rings since the 15th century.

Some climate models suggest that El Niño events will become more frequent and more intense as a result of global warming – potentially causing temperatures to rise further – but this is not certain.

Graphics by the Visual Journalism team.

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